1. Technical Field
The present invention relates in general to a digitally adjustable inductive element. In particular, the present invention relates to a digitally adjustable inductive element which can be utilized to provide a tunable oscillator.
2. Description of the Related Art
In fabricating clock oscillator circuits utilized in digital circuits, it is often necessary to fabricate circuits which are tunable, such as circuits in which the center frequency may be altered. At high frequencies, oscillators based on LC circuits are preferred for reducing jitter and for scaling the power supply voltages. A tunable circuit contains some form of tuning so that the natural resonant frequency associated with the circuit may be varied. In such a circuit, the resonance condition of forced oscillations can be altered. Tuning may be carried out by adjusting the value of the capacitance or the inductance, or both.
An LC oscillator is a type of harmonic oscillator. Harmonic oscillators generate waveforms that are sinusoidal in nature and contain one or more active circuit elements that function to continuously supply power to passive components associated with the LC oscillator. A simple harmonic oscillator is typically composed of a frequency determining device, such as a resonant circuit, and an active element that supplies direct power to the resonant circuit and which also compensates for damping which occurs as a result of resistive losses. The resonant circuit contains both inductance and capacitance arranged in a manner such that the circuit is capable of generating resonant frequencies, depending on the value of the circuit elements and their particular arrangement.
In the case of a simple LC oscillator, application of a direct voltage causes free oscillations in the circuit which eventually decay because of the inevitable resistance in the circuit. Thus, an LC oscillator is essentially a tunable circuit that contains both inductance (L) and capacitance (C). The product, (LC), determines the center frequency of oscillation. The center frequency (.omega..sub.0) is represented by the following equation: ##EQU1##
These type of oscillator circuits are particularly important in synchronizing multiple processors such as those utilized in four-way or eight-way computer systems, well known in the art of digital and computer electronics. For a given computer system to operate properly, each processor must have a substantially identical center frequency. Due to process conditions, one processor clock may differ from another processor clock.
Even if such processor clocks are similar in structure and design, the oscillators upon which such clocks are based must be fine tuned in order to maintain an exact frequency match. Without a substantially exact frequency match, phase slippage results over time. In configurations in which voltage controlled oscillators and LC oscillators are utilized, it is necessary to maintain continuous frequency tuning of the voltage controlled oscillator and the LC oscillator, in order for the configuration to be practical in operation.
There are a number of methods which exist for tuning such circuits. For example, for one-time tuning, tuning can be accomplished utilizing wired fuses or focused ion beam tailoring, techniques well-known in the art. A particular tuning element also well known in the art is a reverse biased diode. When a diode is reverse biased, an associated capacitance is subsequently altered. This capacitance can be part of the C of an LC resonator. Such diodes have a very limited tuning range, usually a range of approximately 15%. When zero voltage occurs across the diode, the depletion capacitance is reduced approximately by an amount represented by the following equation: ##EQU2## In this equation, .O slashed..sub.B represents bulk potential. The limited tuning range implies targeting a center frequency perfectly, which is a difficult task to accomplish. In a very narrow band system that does not include multiple processors, a limited tuning range may be adequate, but for most systems, such a limited tuning range is inadequate.
Based on the foregoing, it can be appreciated that a need exists for a method and system which would allow a user to fine tune the frequency of an LC oscillator in a digital circuit clock. A need also exists to allow a wide range of tuning for such LC oscillators. Because a wide range of tuning is not currently feasible with current devices, applicability is limited to uniprocessor devices. A device, such as the one disclosed herein, not only solves processor problems associated with LC oscillators requiring a wide tuning range, but is also applicable to technological areas outside the processor arena. For example, such a device would also be advantageous in analog situations and technological areas such as wireless and communication networks. A device that allows fine tuning of clock oscillators would be welcomed by those in the industry currently limited in synchronizing circuits driven by current clock oscillator devices.